It is commonly known in the oil and gas industry to use cement compositions to form a sheath in the annulus of an oil or gas well. Cement compositions used in this manner may include cement and any number of additives known in the art. It is also known in the industry that lower density cement compositions (less than about 14 pounds/gallon) are preferable because a higher density cement composition (greater than about 14 pounds/gallon) may fracture the formation.
Gas channeling in a cement composition is a common problem in the oil and gas industry. When a cement composition slurry is first placed in the annulus of an oil or gas well, it is a hydraulic fluid that exerts hydrostatic pressure on the sides of the well. Initially the hydrostatic pressure of the cement composition is great enough to keep gases that are naturally occurring within the reservoir in situ. But as the slurry of cement composition sets, it goes through a transition stage changing from liquid to solid. During this transition stage, the cement composition exerts less and less hydrostatic pressure on the well. It is in this transition stage that the cement composition is susceptible to formation gas entering into the cement sheath. The gas entering into the cement sheath produces pathways filled with gas. As the cement hardens, the pathways become channels in the hardened cement composition, thus the term channeling. Channels in a cement composition weaken the structure of the cement and may compromise the formation of a competent seal within a well.
The advantage of using light weight or lower density cement compositions is that the formation is less likely to fracture than with higher density additives. Light weight or lower density cement is a cement that has a density of less than about 14 pounds per gallon. With lower density cement compositions, it is more difficult to control interstitial water, which is the water between the cement particles. Controlling gas migration to reduce channeling in light weight cement compositions, especially at low temperatures, has been an industry problem for a number of years because the typical additive systems are better suited for heavier or higher density cement compositions.
Presently, the industry typically reduces gas migration using film-forming materials such as SBR latex or polyvinyl alcohol or microparticulate additives such as silica fume in combination with fluid-loss additives to make a cement composition less permeable. These materials work best, however, in cement compositions that have a high cement density and a low water to cement ratio. The lower the cement density and the higher water to cement ratio, the greater the quantity of water soluble or film-forming additives that are required to reduce gas migration to an acceptable level and keep channeling to a minimum. The lower the cement density, therefore, the greater the quantity of traditional additives that are required. This quantity increases to a point that is cost prohibitive for lower density cement compositions.
Controlling gas in light weight cements, especially at low temperatures, has been an industry problem for a number of years because the additive systems that are generally used or employed are better suited for heavier or higher density cements. The additive of this invention presents an alternative that works well with light-weight-low-density cement compositions.